Chabazite-type zeolite is a zeolite having a three-dimensional fine pore structure that includes eight-member oxygen rings forming 3.8×3.8 Å pores. Chabazite-type zeolite occurs naturally, but chabazite-type zeolite having a high silica to alumina molar ratio (hereafter also referred to as “SiO2/Al2O3 ratio”) is exclusively synthesized artificially (see, for example, Patent Literatures 1 to 3, Non-patent Literatures 1 and 2).
Patent Literatures 1 and 2 disclose SSZ-13 as a chabazite-type zeolite having an SiO2/Al2O3 ratio of at least 5. Moreover, Non-patent Literature 1 and Non-patent Literature 2 disclose SSZ-13 in an aggregated form, where crystal particles are bonded to one another chemically. In addition, Patent Literature 3 discloses, under the name SSZ-62, a chabazite-type zeolite having the same crystal structure as SSZ-13, as well as an SiO2/Al2O3 ratio greater than 10 and a crystal size of no more than 0.5 μm.
These chabazite-type zeolites are used as various kinds of catalysts, including cracking catalysts and nitrogen oxide reduction catalysts, and among the applications as catalysts, these chabazite-type zeolites are widely used as nitrogen oxide reduction catalysts that operate by selective catalytic reduction (Patent Literatures 4 to 6). Yet SSZ-13 and SSZ-62 have frameworks that readily break down when exposed to high-temperature, high-humidity atmospheres. Therefore, up to this point, there has been investigation into a chabazite-type zeolite capable of withstanding use as a nitrogen oxide reduction catalyst which operates by selective catalytic reduction.
A chabazite-type zeolite that combines an average crystal size of 1.5 μm or greater and an SiO2/Al2O3 ratio of 15 or greater (Patent Literature 7) has been investigated as an example of a chabazite-type zeolite having a framework that is unlikely to decompose even when exposed to high-temperature, high-humidity atmospheres (that is, having a high level of heat resistance). A chabazite-type zeolite has also been disclosed that, due to crystallizing raw materials that do not include a structure directing agent, has a crystal size greater than 0.5 μm as well as an SiO2/Al2O3 ratio of 5 to no more than 15. This chabazite-type zeolite has a broad size distribution for its crystal particles, the chabazite-type zeolite including crystal particles having a crystal size greater than 0.5 μm to around 3 μm, and is specifically disclosed in scanning electron microscope photographs where these crystal particles are aggregated together (Patent Literature 8).